Two-way telephone system



Sept. 11, 1934. S. B. WRIGHT ET AL 1,973,595

TWO-WAY TELEPHONE SYSTEM Filed June 10, 1931 Pda; L

K ViT""`- M4" .Kindly I Zhzns l I T i' L fi'of- E I l l l s l l l l i IIH; /\/1+ Baita l ,Zea 1 l ATTORNEY i sired over-all eiect.

Patented Sept. 11, 1934 'UNiTED STATES PATENT orties TWO-WAYTELEPHONESYSTEM Application June 10, 1931, Serial No. 543,446

7 Claims.

This invention relates to two-way systems for the transmission ofelectrical waves, which waves transmitl over the system-or, in otherwords, produce at the distant end thereon-some de- More particularly,the invention relates to two-way telephone systems having one or morecomparatively unstable or noisy links and requiring waveresponsivedevices such as singing Suppressors for controlling the 10 transmissionover the transmission paths.

In a telephonesystem, it is often desirable, in order to preventsinging, to have one or both .of -the one way paths disabled at oneormore points. It follows, of course, that means must be provided wherebythe voice waves will cause the removal of any such disability normallyfound on the path over which these waves are to be transmitted. Thus, ifthe incoming or receiving path is normally disabled, it.k is requiredthat the voice waves coming in from the distant end of the system causesome operation at some stage of their travel over the transmissionchannel which will result in rendering the receivingV path operative fortransmission` In addition, as is now well understood in the art, it isoften desirable to have the operation caused by the transmission ofvoice waves extend to the actuation of echo suppressor apparatus fordisabling the opposite outgoing or transmitting path, to prevent thereturn thereover of waves reiiected from the terminal of the system.

Now the transmission system may. include what will be hereinafter termeda mutable link-- that is, a link capable of or liable to change (frominternal or external cause) which may give rise to interfering energy,or, more` specifically, a link speciallyl subject to noise, fading (inthe case of radio. transmission) or change of impedance. For instance,if the system is composed of two wire sections and an intermediateradiolink, the radio link will usually be noisy or otherwise mutable incomparison with the wire sections. The presence of this mutable linkintroduces complications in connection with the Voperation ofwave-responsive devices designed to act on a transmission path on the`(Cl. Z-9) speech. In short, the problem presented is'how to obtainpositive operation of the receiving transmission control devices' inresponse to voice waves without the serious hazard of false operation bynoise energy and the like. Theproper control of the received volumeserves to reduce the problem, but further solution is desirable. Apromising proposed solution'of the problem indicated above involvesk theuse of ,contro1 currents. These currents are single frequency currentsreleased on the transmitting side of the mutable link by the voice wavesand transmitted thereover to the distant or receivingsection of thesystem. In this latter section, suitable means are employed `whereby thewave-responsive defvices such as singing Suppressors respondonly to thecontrol frequency and are thus rendered immune to such energy as noise.As a variation of this solution, the control currentmay be kept on thecircuit when no speech is transmitted and removed under the control ofthe transmitted speech, the operation of the devices at the receivingendbeing altered accordingly. It is Yunnecessary to discuss at anylength herein the respective advantages and disadvantages of the twoforms of special frequency control, but reference may be had to twoUnited States Patents, No. 1,674,695 to Nyquist and Wright and No.1,675,412 to Holden, the latter being directed to the control by specialfrequency of echo suppressors as distinguished from singing suppressors.Y

In addition to the problem stated above, there are other importantproblems which arise in connection with long two-way transmissionsystems, especially in connection with those systems which are equippedwith wave-responsive controlwdevices and which comprise several links,oneV or more of them noisy or otherwise mutable. In long telephonesystems, it is important that all feasible precautions` be taken againstmutual,- lock-out, the condition in which both speakers cause theoperation of wave-responsive devices so as to render the systeminoperative for the transmission of speech in either direction. Again,it is important, when one speaker is temporarilyk locked out by thedevices under the control of the second speaker, that the rstrspeaker beable to break in without undue delay when the second pauses.Furthermore, it is often important that a speaker who takes control ofhis end of the circuit and gets through to the distant end shall be ableto control the incoming path at the distant terminal even though thesecond speaker has started to talk while the waves fromV the vl'lrst arecants have in mind are (l) .therelimination ofthe limitations imposed bystatic in the transoceanic link upon the operation ofthe.wave-responsive devices on the receiving paths leading from that link toLondon and New York, (2) the reduc'- tion of the breaking time and (3)the elimination of mutual lock-out. Thus, in its broader aspect, theinvention resides in methods or in :combinations of -means foraccomplishing allof v'these major results. Other objects and advantagesof the invention will appear upon examination of thelfollowing detaileddescription and discussion of the embodiment Iselected for the purpose'of illustration, which are'to be-read with reference tothe accompanyingdrawing; l v

It will be obviousA that'the invention is equally applicable' tomanyother types of two-way systems including ktransmission media or linksdifferent from those specifically disclosed. For instance, thetransoce'anic radio link might be replaced by a submarine cable withoutreducing the applicability of the principles orthe invention; Figure' lof the drawing shows schematically and in part diagrammatically 'the'western-section of the transatlantic long-wave radio system, connectingNew York with two points in Maine, the western terminals of the radiocircuit. Fig. 2 shows in like manner the eastern sectionof 'the systemwhich connects theY two eastern terminals ofV the radio circuit with iLondon.

The station W1 is the terminal station at New York. It Awill beunderstood that the two-way system may be extendedl westward beyond NewYorkV in any suitable mannen The extension specically indicated isj overa two-wire line L which is connected to the eastward path LE and thewestward pathvLW of a four-wire' circuit through a'hybrid coil, the lineL being balanced by a network. 'The station W2 is at Bangor, from whichpoint transmission over the Atlantic isby radio. The station W3 is atHoulton, at which point the radio waves are received. The path LE isawre path extending from New York to Bangor andy-adapted fortransmission from New York to Bangor. VThe wire ypath LW extends fromI-Ioultonthrough Bangor' to New York and is adapted'for transmission inthat direction. y

At the eastern .end of the system shown in Eig. 2, station El is theterminal station at London, from which the system.may .be extended toEuropean v.points over Vthe A,one-,way line L', for instance, which lineisconnected to the one-way paths LW' and LE' of the .tour-wire circuitthrough a hybridcoil, the line L -being balanced bya network. Station E2is the .radio transmitting: station Yat Rugby, and the line LW isadapted vfor transmission from rstation El to station E2, Station. E3vis at Cuparin Scotland. Thisstation is the receiving terminal of theradio circuit, and the line LE is adapted for transmission betweenstations E3 and E1.

Thus the system extending from station W1 at the west end to station E1at the east end includes a path adapted for transmission from west toeast and comprising the wire section LE, the. radio link. betweenstations W2and E3, and the wire section LE', and another path adaptedfor transmission from east to west and comprising the wire section LW',the radio link between stations E2 and W3 and the wire section LW.

It-'will be'understood, of course, that the principal mutable -link ineither one of these one-way channelsis the radio link. It should beborne in Vmind, however, that wire circuits may be mutable. The sectionsof paths LE and LW lying between .stations W1 and W2 are between 400 and500 miles in length, and the wire connection .between vstations W2 andW3 is between 100 and least, the circuit elements which makeup the "lsystem between the terminals of' the four-wire circuit at stations W1and El, respectively, it -being understood that in many instances theseelements do not, in themselves, for-m any partof the applicantsinvention. A proper understanding of a system such as that specificallydisclosed is essential, however, to the understanding of the invention.Y

Voice waves passing in the line L divide at the hybrid coil, and theeffective portionrof the en- .-l

ergy passes over the line LE through a volume regulator and the one-wayamplier. At the point a, a portion of the energy operates anampliiier-detector 10, which may control relays 11, -12 and 13. The mainportion of the energy passes through ladelay circuit and another one-wayamplifier, through a hybrid coil and a bandsplitting device Pand throughother one-way amplifiers to the station W2. Thedevice P may be anysuitable device for scrambling the speech currents in order tol get adegree of secrecy. The lprinciples upon which such a device operates aredisclosed in United States Patent No. 1,546,439 to Espenschied, forexample.

It will beV understood that the operation of this device involves theproper control of contacts at point g in path LW and point c in path LE,as will be more fully discussed hereinbelow. It should be noted thatbeyond the point c in path LE, provision is made for the applicationtovthat fx path of a current of special frequencyr f1, which is theeastward control frequency. It `ivill` be noted further that the point.at which this conlli() trol current is applied is near the westterminal ci the four-wire circuit, hundreds of A.miles from the radioterminal at station W2.

`Upon reaching station W2, the voice wavesfin path LE pass throughanother volume regulator and other one-way anipliers to the radiotransmitter and .are then radiated from the transmitting antenna A1. Aswill be fullyv described hereinbelow, the transmission path which isnormally disabled at the point e is 'rendered operative for thetransmission of the voice Waves by certain operations under the controlof the special frequency current.

At the European end of the system, the Waves from the antenna A1 atstation W2 are received at station E3 by the antenna Az, are passedthrough the radio receiver and a one-way ampliiier and are thentransmitted to station El, the terminal station At that station, thevoice Waves pass through other one-way amplifiers and a volumeregulator. They also pass through a hybrid coil and a band-splittingprivacy device P which serves to unscramble the voice currents,producing intelligibility. It will be noted that the path LE at' stationEl is normally disabled at the point f. This disability is removed, aswilll be more fully discussed hereinbelow, under the control of thespecial control current transmitted from the American terminal of thesystem. For this purpose, the control energy is diverted from the pathLE at point h to operate the amplifier-detector 40', which, in turn,causes the operation of relay 41Yand also the operation of relays 42 and43. The waves nallypass through the hybrid coil at the eastern terminalof thefour-wire circuit, the eiective portion being transmitted over theextending two-wire line L'.

The Waves passing from east to West are transmitted through the -hybridcoil and the usual one-way ampliers and volume regulatorsin the path LW.A portion of the energy isV diverted to operate amplifier-detector 10',which controls relays 11, 12 and 13. The privacy arrangement is the sameas that indicated in connection with transmission from west to east, thecontacts at points g and c' corresponding to the contacts at the westend. The control current is applied beyond the point c and istransmitted to the American end of the system. This control current is acurrent of special frequency 22. The voice waves which pass the point care transmitted to station E2 and through the radio transmitter and areradiated from the antenna A1.

At station W3, the east-west waves are received in the antenna A2 andthe radio receiver in path LW and are transmittediover the portion ofthe receiving channel at station W2. There certain switching operationsare performed under the control or the east-West control current, aswill be more fully discussed hereinbelow. The east-West Waves are thentransmitted on to station W1, where the control current operatesamplifier-detector l10 and relays 4l, 42 and 43. The Waves pass throughthe usual one-way amplifiers and the volume regulator and also throughrthe hybrid coil and privacy device P, which f unscrambles the Voicecurrents. It should be noted that the east-west path vLW at station W1is normally disabled at pointf and that this disability is removed bythe operation of amplifierdetector 40. At the West terminal ofthefourwire circuit the Waves pass through the hybrid coil and on over thetwo-Wire line L.

It should be noted, in connection with a description of various circuitelements making up the system, that at station E1 between points h and gis a iilter F5. This element is a device for suppressing the controlfrequency f1. Similarly, in the path LW at station W1, between thepoints h and g, there is included a filter Fs. which serves to suppressthe control frequency f2.'

It should be noted further that at station W there is indicated anauxiliary receiving antenna A3, which may be the back end of the antennaA2, and an auxiliary radio receiver. In the output of this auxiliaryreceiver are the relays 51 and 52, which control contacts operating onthe path LW at points Z and Ic. The operation of this'arrangement Willbe disclosed hereinbelow. It Will be noted that no auxiliary switchingdevices are shown at the European radio terminals, stations E2and E3.Fromthis disclosure, it Will be understood that different conditionsdemand diierentarrangements at various points on a two-way system suchas that disclosed, and" that the applicants invention is quiteindependent of many of the details Which it is deemed desirable toinclude in the disclosure of the long-Wave radio telephone system.

It remains to take up in somedetail. the arrangement and operation ofthe various circuit elements with particular regard to the applicantstransmission 'control forming the principal subject matter of theinvention. It should beborne in mind that each one-Waypath at each endof the system is normally disabled near the terminal, i. e. at stationWlor at station El. Path LE is normally disabled at point c, path LW atpoint f, path LW at point c' and path LE at point f. In addition, pathLE is normally disabled at station-W2 at point e for a purpose to bediscussed hereinbelow.

When voice Waves from line L pass over the one-way path LE, there aretwo points in the system at which contacts must be operated to insurethe travel of the Waves over the iirst stage. An examination of Fig. 1will show that the circuit arrangements connected With the privacydevice P are such thatthe circuit is prepared at this point for thetransmission of Waves from east to west in the path LW, the contact atpoint c in path LE being closed to disable the path and the contact atpoint g in path LW being open to render that path operative; in'additionpath LW is disabled at point f. This arrangement, of course, preventssinging. Therefore, rit is necessary for proper transmission from westto east that switching operations be performed inresponse to the voiceWaves in pathLE which Will adapt the privacy arrangement to pass and to'scramble the east-bound 'voice currents and which will insure freepassage of the voice waves toward station W2. This set of operations isperformed by the relays controlled by amplifierdetector 10, into which,it has been seen, a portion of the east-bound voice energy is divertedat point a. UponV the operation of amplifierdetector 10, relays 1l, 12and 13 are operated. Relay 11 removes the disability from `path LE atpointe,` and relay 13 disables the path LW at point g. Accordingly, vthevoice waves Will pass through the hybrid coil and the device P and passon through pointc toward staton W2. It

has been noted that beyond point c an alternating current generator G isassociated with the path LE. When relay 12 operates7 this generator isconnected to the path LE through a filter F1, Which reduces the effectAof transient currents set up by the application of the generator. Thus,the control wave of the frequency fi is sent over path LE somewhat aheadof the voice waves, which, in the circuit under'discussicn, are delayedby the delay circuit between points d and b and by their passage throughthe privacy device P. At station W2, it is desirable that the path LE benormally disabled at point e and that provision be made for thedisabling of path LW at point y', to prevent the lundesirable effects ofradio coupling between `the transmitting antenna A1 and the receivingantennal A2 at, station-.W32 This hazard of radio coupling is, o fcoursegreater as the two radio stations are locatedk closer toeach,other. Accordingly, ,it becomes necessary tojcause some operation whichwill clear the path-LE at point e. This operation the applicants controlby the `control-,current f1, thusvpreventing. false operationby `anynoise currents or, the like originating `in pathLE dus to themutabjllity of the link vconnecting stations W 1 and H A t the point d,the amplifier-detector is connected through a filter F2, which serves topass the control frequency f1 and substantially to exclude otherfrequencies. In'response, then, tothe control wave, which, it hasY Abeenseen, somewhat precedes v*the voice energy, theamplifler-.detectoroperates andpcauses'the 'operationoi relays 21 and 22., Relay 21 removesthe `disability `from path LE at point e, and relay 22 disables the pathLW at point 7'. f

`ln the system specifically disclosed, further measures are takenagainst the hazard of` radio coupling. At station W3 an auxiliary radioreceiver is connected tothe. back endAs Yor the receiving antenna, whichserves to pick up part of the control energy radiated from the antennaA1.. It will be understoody that, by well known methods of directionalselectivity, this auxiliary receiver can be made to respondto thecontrol energy from A1 and be unresponsive to energy coming into stationW3 from Europe.; The auxiliary receiver controls the operation of relays51 and 52. These relays, upon operation, disable the receiving path LWat points Z and k. Thus, the waves transmitted from station W2 areprevented from entering and overloading the radio receiver at station W3and from affecting :the receiving path LW. Further protection againstfalse transmission in the path LW is provided by the operation discussedhereinabove of relay 22 at station W2.

At the European end of thewest-eastY channel, the radio wavestransmitted from station W2 are picked up by the receiving antenna A2and im-v pressed upon the radio receiver. The waves then travel to theterminal station El. At the point h', on transmission path LE', anamplier-detector 40' is connected through a selective circuit Fi, whichrenders the'amplii'ler-detectorresponsive only to the control frequencyf1. When this ampliiier 40 is operated by the control current, relays41', v42 and 43' are operated. VRelay41' removes the normal disabilityat f and allows the passage of the voice waves, which have beenunscrambled in the privacy device P'. Relay 43 disables the oppositepath LW .at b', preventing echoes. Relay 42 serves to prevent theoperation of relays 1-1 and 13 in response to the operation by outgoingvoice waves of amplier-detector 10'; if these relays 11 andl 13 havealready been operated, the operation or" relay 42 terminates theiroperation and transfers from the Londonvspeaker to the New York speakerthe control of the eastern terminal of the system. Thus, if the voicewaves from New York line L have passed control points at station W l andare transmitted through to station El, they can reach the terminalatstation E1 even though the speaker connected to line L' has in themeantime begun totalk and-through the operation of amplier-detector 10has taken temporary control of thenear end of the outgoing path LW; r

As has been indicated hereinabove-the control current of frequency f1ispreventedfrom reaching the subscriber connected .to line L' since itis ,Y blocked by the filter or otherdevice Fs, which isof course, tunedto eliminate .theirequency fi.

The operation of the systemy in Iconnection with transmission from eastto west is similar to that in connection 1 with the opposite'transmission. The voicevwaves from line Lf operate amplifierdetectorvlO,and, if control of this portion of the systemhas not been taken awayfrom the near speaker by the incoming control current,Y relays' 11, 12and 13' are operated. Relays 11 and13 switch the privacy ydevice P',provision thus being made for the scrambling of the outgoing voicewaves,- and the path LW is cleared for transmisf sion. Thefoperation ofrelay 12 applies to the path LW the'east-'west controlcurrentoffrequency f2 from the generatorG through the filter Fi. Due tothe delaysintroduced by thedelay circuit inserted between points a' vand b' and bythe privacy device P',vthis control current somewhat precedes-the voicewaves and is transmitted to station E2, where the radio transmitter andthe antenna Afl serveto transmit thewaves to the American station W3.There the waves are picked up bythe Aantenna A2, and the radio receiveroperates, unless, of course, the auxiliary apparatus has operated todisable the path LW.- Passing over the connecting link to station W2,the incoming control currents are passed through the selective` circuitF3 and operate amplifier-detec torAv 30. The relay 31 controlled b y the`amplifierdetector is then operated and serves to take con-Y trol of thesystem at this point against later arriving waves from New York bypreventing the operation of `relays 21 and 22. In other words, thiseast-west control current serves to maintain the transmitting pathLE indisabled condition 1 even though voice waves are traveling in the opposite direction between stations W1 and W2. This feature eliminatesmutual lock-out since, for instance, if the west-bound energy reachesthis point, control of the incoming path is held regardless of the laterarriving energy traveling in the opposite outgoing path.

The control wave'then passes on to station W1 and operatesamplifier-detector 40, which is connected to the pathLW at point hthrough the 1'.

selective circuit F4, tuned to pass only the control frequency. Relays41, 42 and 43 now operate. The operation of relays 41 and 43 serves toremove the normal disability from the path LW at point f and to 'disablethe path LE at-point. b. The g operation of relay y42 prevents theoperation of relays 11 and 13 in response to voice waves from line L or,if these relays have already been operated, they are promptly releasedin response to .the'operation of relay 42. Thus, as has been seen inconnection with the opposite end of the system, the westward-boundenergy reaching station W1 assures control of the circuit even thoughthe New York speaker has begun to talk. When relay 41 operates, removingthe disability from i path LW at point f, this `path is cleared for thepassage of the voice waves, which now travel freely gto the listenerconnected tol line L.

It will now beunderstood that, by virtue of the applicants arrangements,the normally disabled l 1.

rIjhe control wave, it will be understood by those skilled in the art,in addition to beingconned to fa narrow frequency band and thus easy todetect in spite of interfering noise energy, is applied suddenly withfull amplitude and is thus advantageous over voice waves for theproductionof the prompt and positive operation of the transmissioncontrolling relays.

It will beV noted, further, that in their travel over the system theunassisted voice waves are required to operate only one set ofrelays-those controlled by the amplifier-detector 10 (or 10'); as thewaves pass on over the system, all subsequent operations areaccomplished by the control energy. Accordingly, thereis no additionalopportunity for clipping of the voice waves after the rst operation,even though relays must be operated at several successive points topermit the passage of the voice waves.

Again, by virtue of the applicants arrangements, the waves which passover the system to vthe distant end are given positive control of theincoming channel at that end, even though the waves passing in the otherdirection have already entered the outgoing path; thus the breaking timeis reduced to the minimum.

Furthermore, it Willbe noted that the chances of mutual lock-out iseliminated, since the subscriber who rst takes control of relays 21 and22 at station W2 will be heard by the subscriber connected to the otherend of the system.

In addition, there is provided at one terminal of the radio linksatisfactory prevention of radio coupling. It is, of course, feasible toprovide this rfi feature at both ends of the radio link, if such anarrangement is desirable.

While the invention has been disclosed in detail in our specificembodiment, it is to be understood that such disclosure is merely forthe purpose of illustration and that the invention may be embodied inmany other and Widely different forms within the scope of the appendedclaims.

What is claimed is: 1. In a two-way system for the transmission ofelectrical waves, including a terminal station, a

radio transmitting station, a radio receiving station, a transmittingpath connecting said terminal station and said transmitting station anda receiving path connecting said receiving station and i said terminalstation, the method of transmission control which consists in causingthe waves in said transmitting path at the terminal station to applyspecial control energy to said path for transmission to the transmittingstation, radiatfing said control energy from the transmitting stai; saidtransmitting station, a receiving path connecting said receiving stationand said terminal station, means at said terminal station responsive tothe waves transmitted to produce the desired over-all eiect forcontrolling the transmission I; over said transmitting and receivingpaths thereat and for applying special control energy to saidtransmitting path, and means responsive to said control energy forpreventing radio coupling between said transmitting and receivingstations.

3. In a two-Way system for the transmission of electrical waves,including a radio link, two terlink to the corresponding terminalstation, means associated with the terminal end of one of saidtransmitting connections and responsive to the wavesk transmittedthereover for applying control energy to said connection, meansassociated with the terminal end of the distant receiving connection andresponsive substantially only to said control energy for controlling thetransmission over said receiving connection and the oppositetransmitting connection, and means responsive to the control energyapplied to said transmitting connection for preventing radio couplingbetween the radio end of said transmitting connection and the radio endof the opposite receiving connection.

4. In a two-way system for the transmission of electrical waves,including a radio link, two terminal stations each distant from one endof said radio` link, a transmitting connection and a receivingconnection from each end of said vradio link to the correspondingterminal station,.mean`s associated with the terminal end or one of saidtransmitting connectionsi and responsive to the waves transmittedthereover for applying control energy to said connection, meansassociated With the radio end of said transmitting connection andresponsive substantially only to said control energy for controlling thetransmission over said transmitting 4connection and the oppositereceiving connection, means responsive to the control energy applied tosaid transmitting connection for preventing radio coupling between theradio end of said transmitting connection and the radio end of theopposite receiving connection, and means associated with the terminalend of the distant receiving connection and responsive substantiallyonly to said control energy for controlling the transmission over saidreceiving connection and the opposite transmitting connection.

5. In a two-way system for the transmission of electrical waves,including two paths adapted for transmission in opposite directions, aradio link forming an intermediate portion of each of said paths, twoterminal stations each distant from one end of said radio link and atransmitting connection and a receiving connection from each end of saidradio link to the corresponding terminal station, the method oftransmission control which consists in causing the waves transmittedover the terminal end of one of said transmitting connections to applycontrol energy lto said transmitting connection, radiating said controlenergy from the radio end of said transmitting connection, causing aportion of the radiated control energy to be received at the radio endof the opposite receiving section at the same end of the radio link,causing the control energy so received to disable the radio end of saidreceiving connection for any wave transmission, and causing the controlenergy reaching the terminal end of the distant receiving connection onthe other side of the radio link to control the transmission over saiddistant receiving connection and the opposite transmitting connection.

6. In a two-way system for the transmission of electrical waves,including two paths adapted for transmission in opposite directions, aradiolink forming an intermediate part of each of said paths, twoterminal stations each distant from one end of said radio link and atransmitting conille minal station, the method vof transmission con-`trolivvhich consists in causing the Waves transmitted over the terminalend of one of said transfmitting connections to control the transmission.over said co-nnection andthe opposite receiving connection and to applycontrol energy to said transmitting connection, radiating saidr controlenergy from the radio end of said transmitting connection, causingY a'portionf'of thev radiated control energy to be received at the radioAen'd of the opposite receivi'ngconnection at the same end of the radiolink; causing the control energy so received to disablethe' oppositereceiving connection at said end of the' radio link for any Wavetransmission, and causing 'the control energy reaching the terminal' endof Athe distant receiving connection to control the transmission oversaid vdistant receiviigconnecti'o'n and the oppo- .site transmittingconnection.

7. In a two-waysystem'for the trarisnis'sion or electrical Wavesincluding two paths adapted for transmission in opposite directions; aradio link forming an intermediate portion of each of said paths, twoterminal stations each distant from one Vendv of said radio link` and atransmitting ymitting connection.

nection and a receivingconnectiony from 'each end' of said radio link tothe correspondingter- 'at' the 'saineend'ef theradio link, causing thecontrol energy so: received to disable the radio end of said `receiving'connectionffor any Wave transmission, cansing'the' control 'energyreceived at th'eterrninal end'oi eitherre'cei-Ving connection to controlthe transmission' over the near ends of the"p`aths`,1a'nd 'causing thecontrolenergy receivedat the'terminalendof one of said receivingconnections from the distant end of the radio link vtorenderthe nearends of the paths exempt from transmission controlbyenergy in the neartrans- SUMNER B. WRIGHT. EDMUND TAYLOR.

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